A glider is a special kind of aircraft that has no engine.
Paper airplanes are the most obvious example, but gliders come a wide
range of sizes. Toy gliders, made of balsa wood or styrofoam, are an excellent
way for students to study the basics of
aerodynamics.
The Wright brothers perfected the design of the first airplane and
gained piloting experience through a series of
glider flights.
These aircraft were similar to modern hang-gliders and were
launched by running off a hill.

If a glider is in a steady
(constant velocity and no acceleration) descent, it loses altitude as
it travels. The glider's flight path is a simple straight line,
shown as the inclined red line in the figure. The flight path
intersects the ground at an angle "a" called the glide angle.
If we know the distance flown and the altitude change, we can
calculate the glide angle using trigonometry.
The tangent (tan) of the glide angle (a) is equal to the change in height (h)
divided by the distance flown (d).

Considering the
vector balance of forces for a glider, we
find that the lift (L) times the cosine (cos) of the glide angle plus
the drag (D) times the sine (sin) of the glide angle is equal to
the weight (W) and that the lift times the sine of the glide angle
is equal to the drag times the cosine of the glide angle.
With these three equations we can experimentally determine the
lift and drag of a glider.
If we test fly a glider (paper airplane) and measure the
distance along the ground, the height at which the glider is
released, we can use the first equation to give us an average value of the
glide angle. With the glide angle known and a measurement of the weight,
we have two remaining equations in two unknowns which can be solved for
the lift and the drag.